[rc5] key security question

Fedor Kouranov ted99 at ibm.net
Thu Jun 5 16:39:18 EDT 1997


On 06/05/97 Benedikt Eric Heinen <beh at icemark.ch> said:

>  - Is there still a single 56bit key that can decrypt the file (i.e.
>    the second encryption doesn't give any additional security)?

No, if you have 2 keys, you have 2 keys. The first pass *might* interfere
somehow with the second so that cracking the second key will be easier, but
that's extremely unlikely.

>  - Do the bits just add up (i.e. the resulting ciphertext will be 
>    like it was encrypted using a single 112 byte key)?

Think keyspace: for each first key you have to search all the 2^56 second
keys. This makes 2^112 combinations, IOW a 112-bit key. This presumes that
using double keys does not weaken encryption (as in previous paragraph).

>  - Do the keys multiply (i.e. the result would be like it was encrypted
>    using a 56^2 bit key)?
>  - Will the keys power each other (56^56 bit protection)?

No, that's too much, especially the last one ;-) Nobody will have to search
a bigger keyspace than was originally. Remember that the relationship
between key length and keyspace size is exponential (keyspace =
2^keylength).

And, anyway, you don't need all this stuff because you can simply have 128
bit RC5 which will give you the protection you wanted.

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